1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine, or a facsimile machine, and more specifically, it relates to an image forming apparatus that transfers a toner image formed on an image bearing member onto a recording material using a transfer belt or an intermediate transfer belt.
2. Description of the Related Art
An image forming apparatus that uses an electrophotographic method in the process of forming an image uses, as a belt member, a transfer belt that bears and conveys a recording material, or an intermediate transfer belt that bears a toner image transferred from a photosensitive member. In such a configuration, the transfer belt or the intermediate transfer belt is looped over a plurality of rollers, and the displacement of the belt is controlled (Japanese Patent Laid-Open No. 09-48533). To control the displacement of the belt member, a sensor is provided that detects the position and speed of the belt member. When the belt member is displaced or meanders, the inclination of the shaft of at least one of the rollers is changed to correct the displacement. Alternatively, a displacement limiting member is provided along the edge of the belt member to limit the displacement of the belt member.
In each case, the belt member is stressed in a direction perpendicular to the rotating direction, and a ridge or groove (hereinafter referred to as tension line) can be generated in the belt member along the conveying direction (Japanese Patent Laid-Open No. 2004-252300).
If a tension line is generated, a gap is generated between the image bearing member and the belt in the primary transfer section. Therefore, when a transfer bias is applied, due to this gap, there is a difference in resistance of the primary transfer section between the tension line and other regions. When there is a difference in resistance, there is a difference in transfer current. This causes defective transfer such as a weak current white spot or an excess current white spot in the tension line. As a result, defective image formation (a vertical line) attributed to the tension line is caused.
In a normal image forming apparatus, causes of a vertical line include, in addition to a tension line, contamination of a primary charging wire or a charging roller, contamination of a developing roller, and defective cleaning.
A vertical line due to defective cleaning is relatively easily identified because it is generated in the sub-scanning direction regardless of electrostatic latent image, that is, regardless of whether image part or non-image part.
However, causes of a vertical line include not only defective cleaning but also a tension line of a transfer belt or an intermediate transfer belt, contamination of a primary charging wire or a charging roller, and contamination of a developing roller. When a vertical line image is generated, it is not easy to identify which component causes the vertical line just by viewing the image.
A tension line generated in an intermediate transfer belt or a transfer belt causes a vertical line image even if the tension line is about 2 μm in depth and about 2 mm in width. Therefore, it is difficult to visually identify the location of a tension line. The same can be said to some extent for contamination of a primary charging wire or a charging roller.
The time of occurrence of damage or contamination of each component that causes a vertical line depends on use environment. The damage or contamination can occur by chance. The time of occurrence of the damage or contamination cannot be predicted from the life of each component.
Replacing all components including normal components when defective image formation occurs increases the cost.
To prevent the increase in cost, components can be replaced one by one. Every time a component is replaced, a test is conducted to determine whether defective image formation still occurs. Thus, the problem component can be identified.
However, unlike a charger and a developing unit, a belt member cannot be easily replaced. The above-described method includes many processes and takes a lot of time.
It is desirable to determine whether a tension line is generated in a belt member by a simple method.